Monoamine oxidase (MAO) is an enzyme that oxidizes monoamine neurotransmitters and neuromodulators, as well as exogenous bioactive monoamines. It was first characterized by Hare in 1928 and was later called MAO by Zeller in 1938. Following the characterization of this enzyme, it was later discovered that its inhibition could have positive effects on psychiatric disorders such as depression.
Iproniazid, described in the late 1950's and used as a treatment for tuberculosis, was found to have mood-elevating properties. It was later shown to be a suitable MAO inhibitor and was used thereafter as an effective antidepressant. However, the drug had to be withdrawn from the U.S. market in the early 1960's because of the reports of hepatic toxicity and occasional hypertensive crises associated with its use. Still, the success of Iproniazid as an antidepressant stimulated pharmaceutical companies to search for new MAO inhibitors having antidepressant properties without adverse side effects. Since then, a large number of MAO inhibitors have been synthesized and administered.
Until 1972, when it was discovered for the first time that MAO existed in two forms, namely MAO-A and MAO-B, the first generation of MAO inhibitors had no selective inhibitory activity towards MAO-A and/or MAO-B. Examples of these compounds are the drugs phenelzine and tranylcypromine, respectively patented in 1959 (U.S. Pat. No. 3,000,903) and 1961 (U.S. Pat. No. 2,997,422). Apart from inhibiting the activity of both MAO-A and MAO-B, these non-selective irreversible MAO inhibitor antidepressants also exhibit other important drawbacks. Hence, these drugs have been categorized as "dirty" drugs. In other words, they also block other enzymes and most importantly, they can, similarly to Iproniazid, cause severe hepatotoxicity and hypertension resulting from the ingestion of tyraminerich food and drinks. This is caused by the fact that dietary amines are not broken down after ingestion and thus release circulating catecholamines which may lead to hypertensive crises and sometimes death. Thus, non-selective MAO inhibitors of this type have acquired a bad reputation and although they are very effective antidepressants, they have been avoided by most psychiatrists in favour of the relatively safer tricyclic antidepressants.
In the mid 1960's, a French group headed by Jacques R. Boissier published data on the synthesis of three series of new aliphatic and cycloaliphatic derivatives of hydrazine, propargylamine and cyclopropylamine, suspected to be useful as monoamine oxidase inhibitors (Chimie Therapeutique (1966), 320-326). Boissier et al. suggested that these non-selective total MAO inhibitors might possess therapeutic properties for the treatment of depression or angina pain. In French Patent 1,453,844, N-propynylalkylamines having a linear or branched alkyl group of 6 to 9 carbon atoms on the amino moiety are described.
In a further 1967 publication (Therapie, XXII, 1967, 367-373), Boissier et al. reported the results of tests conducted with these compounds to evaluate their antidepressant activity. Based on the results obtained, Boissier et al. concluded that the aliphatic compounds of the propargylamine series were practically inactive in vivo, regardless of whether the amine was secondary or tertiary, and only moderately active in vitro. From these results, it seemed that a promising future could not be foreseen for aliphatic propargylamines as effective MAO inhibitors. Hence, research involving compounds of this type was completely abandoned after the 1965, '66 and '67 publications by Boissier et al. It turned out that most of the research done later on MAO inhibitors concentrated on aromatic compounds.
In the early 1970's, it gradually became apparent that MAO existed in multiple forms, namely MAO-A and MAO-B. These two types of enzymes have been found to be somewhat different from one another. They exhibit different substrate profiles, they respond differently to selective inhibitors, they are found in different cellular and subcellular locations and they are distributed differently between neuronal and non-neuronal structures. Recently, MAO-A and MAO-B have been shown to arise from different gene loci. MAO-A is located predominantly inside the neurones and is responsible for causing hypertensive crises. It preferentially deaminates and oxidizes 5-hydroxytryptamine. As for MAO-B, it is found mostly in glia and it preferentially oxidizes .beta.-phenylethylamine.
The discovery of MAO-A and MAO-B was of major importance since it initiated the research that led to the synthesis of second generation MAO inhibitors. The second generation MAO inhibitors are compounds that irreversibly or reversibly inhibit either the A or the B form of the enzyme. Because both the antidepressant and hypertensive effects are considered to be related to the inhibition of MAO-A, drug companies have concentrated their efforts mainly in the development of MAO-A inhibitors. Clorgyline, Lilly 51641 and PCO were among the first selective MAO inhibitors for MAO-A to be discovered. All these compounds belong to the first category of second generation MAO inhibitors and form irreversible links with the A enzyme.
The reversible specific MAO inhibitors, which form the second category of second generation inhibitors, have recently attracted attention because of their potentially improved clinical properties. Included in this category are harmine, harmaline, cimoxatone, brofaromine, amiflamine and moclobemide.
In recent years, a MAO-A inhibitory prodrug has also been discovered. MDL-72394 can be decarboxylated by aromatic L-amino acid decarboxylase and forms a potent irreversible MAO-A inhibitor, which has been shown to be neuronal selective. The chemical structures of first and second generation aromatic MAO-A and -B inhibitors may be found in Chapter 7 of Neuromethods, Volume 5, Neurotransmitter Enzymes, 1986, Humana Press, the contents of which is hereby incorporated by reference.
Research on MAO-B inhibitors is nowhere near the level of research accomplished so far for MAO-A. In fact, only a few irreversible MAO-B inhibitors such as Deprenyl and Pargyline have so far been discovered. Deprenyl is one of the most important and widely tested MAO-B inhibitors. It has been used as an effective adjuvant to L-DOPA in the treatment of Parkinson's disease. The combination of Deprenyl and L-DOPA seems to reduce the requirement for L-DOPA (presently known to be the best antiparkinsonian agent) in those cases where L-DOPA is being ingested. Recently, it was reported that Deprenyl alone can significantly delay the onset of disability associated with early, otherwise, untreated cases of Parkinson's disease. It has also been claimed that the use of Deprenyl improved the clinical condition of some Alzheimer's patients and reduced depression, attention deficit disorders and potentially other neuropsychiatric disorders. In addition, Deprenyl has been observed to prolong life span and sexual activity in animals and humans. Unlike MAO-A inhibitors, MAO-B inhibitors do not usually cause hypertensive crises except, in some instances, under chronic large-dose applications and therefore have the potential to become very useful neuropsychiatric and geriatric drugs.
Although Deprenyl at higher doses can cause a slight increase in dopamine levels in the brain, the involvement of dopamine in the mechanism of action of Deprenyl has not been well established. The inhibition of MAO-B activity causes a selective accumulation of .beta.-phenylethylamine, a typical MAO-B substrate, which is present endogenously, including in the central nervous system. .beta.-Phenylethylamine, which possesses stimulant properties, can amplify dopaminergic function and modulate dopaminergic neurotransmission and is therefore related to the chemotherapy of MAO-B inhibitors.
It was also found that since Deprenyl is a structural analog of amphetamine, it is catabolized to produce small amounts of amphetamine. This has caused some concern because it was hypothesized that Deprenyl might, in some instances, be a drug subject to substance abuse. Hence, different MAO-B inhibitors not possessing amphetamine-like properties are required. Recently, the reversible MAO-B inhibitors MD 780236 and RO-16-6491 as well as the irreversible inhibitor MDL-72145 were discovered but other alternatives are still being sought. Recent studies on currently available MAO-A and MAO-B inhibitors are summarized in Youdim et al., (1991) Biochemical Pharmacology, Vol. 41, No. 2, pp. 133-162, which is hereby incorporated by reference.
In 1989, the results of a systematic investigation on the deamination by MAO-A and -B of amines having aliphatic chains of various lengths were published (J. Pharm. Pharmacol. 1989, 41:205-208). It was found that these amines were readily oxidized by MAO-B with very high affinity. The deamination of these aliphatic amines by MAO-B was found to be even more sensitive to Deprenyl than that of .beta.-phenylethylamine, which is known to be a typical MAO-B substrate. Unfortunately, although these compounds were found to be good substrates for MAO-B, they did not exhibit any inhibitory activity towards this enzyme.
In summary, active research on MAO inhibitors has been carried out since as early as 1950 and hundreds of potentially useful MAO inhibitors have been synthesized. There was an important change in research focus in the early 1970's when the existence of two different forms of MAO enzymes was discovered. It seems that substantial progress has been made in MAO-A inhibition but much more work remains to be done to find suitable MAO-B inhibitors. Since the inhibition of MAO-B appears to alleviate the symptoms of aging associated diseases such as Parkinson's disease and Alzheimer's disease, suitable MAO-B inhibitors would be highly desirable, especially in view of the limited and relatively inefficient treatments available for these diseases.
The central nervous system, particularly the dopamine system, has received considerable attention in the field of age-related neuronal degeneration and neurodegenerative conditions, such as Parkinson's disease, Alzheimer's disease, etc. Several neurochemical markers of the brain's dopaminergic system, such as dopamine levels, activity of the key enzyme tyrosine hydrolase, densities of dopamine receptors and the dopamine uptake system are all found to be reduced in the normal aging process (Morgan and Finch, Ann. N.Y. Acad. Sci. 515 (1988):145-60) and in neurodegenerative disorders. Changes to these markers are the result of neuronal death in specific regions of the brain. These neuronal losses are irreversible and the intensity of the damage increases with age. The cause of the cell death is unknown. When dopamine neurone numbers are reduced to about 20% of controls in the striatum, for example, pathological movement disorders begin to appear (i.e. Parkinson's disease). Although the symptoms of this disease can be treated with 1-dopa, it unfortunately is only beneficial for a limited period of about three to five years. Cell death and neuronal degeneration continues in a progressive manner and several hypotheses regarding this have been proposed. MPTP was found to cause Parkinson's disease (Langston, Science 225 (1984):1480-1482) and it has been suggested that the disease could perhaps be caused by MPTP-like substances to which patients have been exposed or that perhaps these substances could be generated endogenously (Snyder and D'Amato, Neurol. 36 (1984):250-258). MPTP is converted by MAO-B in the brain to MPP.sup.+, which is considered to be a distal toxin. Blocking MAO-B activity, therefore, prevents neurons from damage by MPTP-like neurotoxins. In one animal study deprenyl has been shown to protect dopamine neurons even after the MPTP has been completely washed out of the tissues (Tatton, 3rd Can. Conf. Neurodegenerative Diseases, PD, (abstract) Toronto). The mechanism of this neuroprotective effect has yet to be determined.
Neuronal degeneration may be caused by an increase in oxidative stress derived from MAO-catalyzed oxidative deamination of dopamine and other amines. In these reactions, hydrogen peroxide is produced as a side product. In the presence of metal ions, such as ferrous, hydrogen peroxide is converted to hydroxyl free radical, an extremely reactive substance, which causes lipid peroxidation and subsequent harmful oxidative chain reactions. Such reactions cause damage to the cellular components of cells, particularly mitochondrial membranes, and thus they destroy neurons. The produced toxic and harmful oxygen peroxide can be detoxified in various ways, such as removal via reaction with reduced glutathione (GSH) to produce oxidized glutathione (GSSH) or by catalase (H.sub.2 O.sub.2 .fwdarw.H.sub.2 O+O.sub.2) or by peroxidase (R(OH).sub.2 +H.sub.2 O.sub.2 .fwdarw.H.sub.2 O+RO.sub.2). It is of interest to note that both H.sub.2 O.sub.2 and oxidized glutathione are found to increase with ageing (Sohal and Allen, In The molecular basis of ageing, pp 75-104, eds Woodheat et al, Plenum Press, N.Y. 1985). Iron has been shown to be elevated in the postmortem brain of PD patients (Dexter et al, J. Neurochem. 52 (1989):1830-1836). One cannot simply administer Fe chelators, however, as a way to reduce neurodegeneration because of the possibility of serious unwanted side effects. Iron is an essential cofactor in many vital enzymes. MAO-B activity is known to be elevated in ageing brain (Fowler et al, J. Neural Transm. 49 (1980):1-20) and this suggests that the ageing brain suffers oxidative stress due to excessive deamination. Reduction of such stress (i.e. caused by H.sub.2 O.sub.2) by inhibiting MAO-catalyzed deamination reactions, might seem therefore rational as a treatment to reduce oxidative stress and neuronal deterioration.